1. Field of the Invention
The present invention relates to a pressure roller suitable for use in a fixing device mounted in an image forming apparatus, such as an electrophotographic copying machine and an electrophotographic printer, and a fixing device including the pressure roller.
2. Description of the Related Art
As fixing devices mounted in electrophotographic printers or electrophotographic copying machines, there has been known a heat roller-type fixing device including a halogen heater, a fixing roller heated by the halogen heater, and a pressure roller brought into contact with the fixing roller to form a nip portion.
Additionally, there has been known a film heating-type fixing device including a heater having a heat generating resistor formed on a substrate made of ceramics, a fixing film moving on the heater while being brought into contact with the heater, and a pressure roller forming a nip portion together with the heater via the fixing film.
Each of the heat roller-type fixing device and the film heating-type fixing device is configured to heat and fix a toner image onto a recording material carrying an unfixed toner image thereon while the recording material is pinched and transported at the nip portion.
A releasing layer is generally provided on a surface layer of the fixing roller or fixing film and a surface layer of the pressure roller, which are used in these types, to prevent the toner from adhering thereto. A fluorine resin can be used as the releasing layer.
However, since the fluorine resin is a high electrical insulation material, the fluorine resin has properties in which it is easily electrically charged and static electricity is hardly escaped therefrom. For this reason, if the recording material with the unfixed toner image is transported to the nip portion of the fixing device, an electrostatic offset image (hereinafter, referred to as electrostatic offset) is likely to occur, in which the unfixed toner electrically adheres to a surface of the fixing roller or fixing film and is then fixed to the recording material when the fixing roller or fixing film revolves.
Accordingly, a method of dispersing a charge control agent onto the fluorine resin or a method of applying a voltage to the pressure roller has been proposed to prevent the fluorine resin of the releasing layer from being electrically charged.
For example, Japanese Patent Application Laid-Open No. 04-19687 discusses a fixing device including a fixing member having a heat generating element therein, and a pressure roller placed opposite to the fixing member in a freely rotating manner, in which the pressure roller has an electrically conductive core metal, an elastic layer formed on the core metal, and a surface layer of an electrically conductive PFA tube formed on the elastic layer.
Furthermore, Japanese Patent No. 3,102,317 discusses a pressure roller and a fixing device characterized in that the pressure roller includes an insulating surface layer formed on an outermost layer of the pressure roller, and at least one low-resistance layer formed inside the insulating surface layer and applied by a voltage, and the lateral surfaces of both ends of the pressure roller are coated with insulating material.
In addition, Japanese Patent Application Laid-Open No. 2008-222942 discusses a fluorine resin composition containing a fluorine resin, a fluoroalkylsulfonate, and no conductive particle, which is applied to a copying machine or a printer.
However, the configurations of the related art have the following issues from the viewpoint of electrostatic offset and toner stain.
First, the electrostatic offset and the toner stain will now be described herein. When a surface potential of the pressure roller is excessively increased by frictional electrification when an electric field to attract the toner to the surface of the fixing film or fixing roller from the recording material is generated, and thus the electrostatic offset occurs on the recording material. Therefore, an offset image is continuously produced on the whole image. Meanwhile, the toner stain is the one in which the offset toner adheres to and accumulates on the surface layer of the pressure roller. A lump of toner adheres to the underside of the recording material at any timing, which causes an image defect.
In the case of the related art where the conductive PFA tube is provided as the surface layer of the pressure roller, the toner stain easily develops on the pressure roller. The conductive PFA tube is made by adding carbon to insulating PFA to produce conductivity. As compared with the insulating PFA tube with no conductive material, its electrostatic offset is superior, while its releasing property of the toner is inferior.
If the content of the carbon is reduced, the releasing property is improved, but the electrostatic offset is decreased. Accordingly, in the carbon-added conductive PFA tube, the electrostatic offset and the stain of the pressure roller are in a trade-off relationship.
In addition, in the configuration in which the insulating PFA tube is used on the surface layer of the pressure roller, and at least one low-resistance layer is formed inside the insulating surface layer and applied by a voltage, the applied voltage needs to be very high. The reason is that the surface layer of the pressure roller is frictionally charged by feeding paper, and thus it is necessary to supplement the surface potential of the pressure roller, which is strongly shifted to negative polarity, by applying a voltage. In this instance, leak caused by partial electrical breakdown or the like is likely to occur on the surface of the PFA tube. Furthermore, although the electrostatic offset and the stain of the pressure roller were examined by applying a voltage while the content of the carbon was gradually reduced, an improvement in the electrostatic offset is not compatible with a reduction of the stain of the pressure roller.
Meanwhile, a tube including fluoroalkylsulfonate contained in the fluorine resin (PFA) has a tendency to improve the frictional electrification property with paper, as compared with the insulating PFA tube, but does not exhibit an effect on the electrostatic offset in the case of paper that easily causes the pressure roller to be charged.